Wood is commonly used in construction and furniture, and many users pay a premium for clear, defect-free wood. A number of chemical agents and natural processes can discolor and deteriorate wood, reducing its aesthetic and structural value. Discolorations or "sapstains" are abnormal color patterns that develop in wood and adversely affect its value. Most discolorations are readily delineated from normal color patterns. Discolorations that develop in the sapwood of various species of wood during lumber manufacture and drying have been a continuing cause of financial loss to the forest products industry. Losses due to discolorations have become greater in recent years because of an emphasis on natural finishes and the growth of an export market for clear, light wood timbers. R. A. Zabel and J. J. Morrell, "Wood Stains and Discolorations," Wood Microbiology: Decay and Its Prevention, pp. 326-43 (1992) (hereinafter "Wood Microbiology").
Compounds and treatments have been developed for preserving wood and wood materials having good activity against wood-discoloring fungi and wood-destroying insects. See, e.g., U.S. Pat. No. 5,196,407. Treatments have also been developed to control wood discoloration caused by microorganisms. Treatments used to control discolorations caused by microorganisms in wood, however, are ineffective in preventing discoloration caused by wood enzymes. L. H. Williams, et al., "Treatment of Freshly Sawn Hardwood Lumber," Proceedings: XIX Annual Hardwood Symposium of the Hardwood Council, Starkville, Miss. (March 1991) pp. 105-117; J. W. Clark, "A Gray Non-fungus Seasoning Discoloration of Certain Red Oaks," Southern Lumberman 194:35-38 (1957).
Dark discolorations caused by enzymatic reactions have been seen in softwoods such as ponderosa pine, white pines (eastern, western and sugar) and western hemlock after kiln drying during the lumber manufacturing process. E. E. Hubert, Outline Of Forest Pathology (John Wiley, New York, 1931). G. M. Barton and J. A. F. Gardner, "Brown-Stain Formation and the Phenoiic Extractives of Western Hemlock," Publication No. 1147, Department of Forestry, Ottawa, Canada (1966). The stain develops both in the sapwood and in the heartwood. The enzymes oxidize phenols to leuco intermediates. These compounds subsequently react with oxygen and are carried toward the wood surface during kiln drying to form a brown stain. M. A. Hulme and J. F. Thomas, "Control of Brown Stain in Eastern White Pine with Reducing Agents." Forest Prod. J. 33:17-20 (1983).
A similar stain has been noted on Douglas fir. This stain develops rapidly under moist, warm conditions. D. J. Miller et al., "Chemical Brown Staining of Douglas Fir Sapwood," Forest Products Journal 33(4):44-48 (1983). Water-soluble extractives appear to migrate to the wood surface, where they undergo oxidation to produce a brown, polymerized pigment. The stain usually is close to the surface of the wood, but also has been observed deeper in bulk-piled wood.
Hardwoods can develop deep yellow to reddish brown discolorations on the surface of the wood when exposed to air immediately after sawing or peeling. These discolorations are especially noticeable on cherry, birch, red alder, sycamore, oak, maple, and sweet gum. This stain develops in red alder, oaks, birch, and maple during air-seasoning. Wood Microbiology pp. 326-43. Such stain is absent in wood that is immediately kiln dried and appears less frequently under cool conditions. The staining is most likely caused by an enzymatic reaction. I. W. Bailey, "Oxidizing Enzymes and Their Relation to "Sap Stain" in Lumber," Botanical Gazette 50:142-147 (1910).
A related gray stain on several varieties of southern oaks also appears to be oxidative in nature. J. W. Clark, "A Gray Non-fungus Seasoning Discoloration of Certain Red Oaks," Southern Lumberman 193(2417):35-58 (1956). Gray stain is more difficult to control in lumber cut from water-stored logs than from freshly cut logs. This problem may be caused by the decreased senescence of parenchyma cells in water-stored logs compared to dry-stored logs. P. G. Forsyth and T. L. Amburgey, "Prevention of Non-microbial Sapstains in Water-stored Oak Logs," Forest Prod, J. 42(4):59-61 (1992).
Methods for controlling enzyme-mediated discoloration of wood have centered on the use of heat, such as steam, hot water, or microwave energy. The stain in alder and several birches can be controlled by immersion in boiling water for several minutes. I. W. Bailey, "Oxidizing Enzymes and Their Relation to "Sap Stain" in Lumber," Botanical Gazette 50:142-147 (1910). This method of stain prevention is slow and costly and therefore is rarely used.
Additional methods of preventing enzyme-mediated staining include kiln-drying of unseasoned lumber as soon as it is cut or use of long, mild kiln schedules. These remedies, however, are impractical and therefore rarely used. Another method involves dipping of individual pieces of milled lumber into solutions of various reducing agents or anti-oxidants in order to control enzyme-mediated discoloration. T. L. Amburgey and P. G. Forsyth, "Prevention and Control of Gray Stain in Red Oak Lumber," Proceedings, Hardwood Research Council pp. 92-99 (1987); M. Y. Cech, "New Treatment to Prevent Brown Stain in White Pine," Forest Prod. J. 16(11):23-27 (1966); P. G. Forsyth and T. L. Amburgey, "Prevention of Non-microbial Sapstains in Water-stored Oak Logs," Forest Prod. J. 42(4):59-61 (1992). Freshly sawn lumber is dipped in sodium azide or sodium fluoride to prevent brown stains in some types of wood.
This dipping in sodium azide or sodium fluoride is not favored, though, because these compounds are hazardous. The use of these chemicals is undesirable given their environmental and in-plant pollution potential and residual presence on wood provided to the consumer. Further, these chemicals are not completely effective against gray stain, another enzyme-medicated discoloration of hardwoods.
A one-dip treatment using ammoniacal zinc oxide was found to be as effective as dipping treatments of sodium fluoride when eastern white pine was treated at a lumber mill to arrest stain appearing during kiln-drying or air-seasoning. This solution could effectively control brown chemical staining (kiln-burn) occurring during kiln-drying. M. A. Hulme and J. F. Thomas, "Stain Control in Eastern White Pine Using Ammonical Zinc Oxide in Mill Conditions," Forest Prod. J. 25(6):36-39 (1975); M. A. Hulme and J. F. Thomas, "Control of Brown Stain in Eastern White Pine with Reducing Agents," Forest Prod. J. 33(9):36-39 (1983); E. W. Price, "Chemical Stains in Hackberry Can Be Prevented," Southern Lumberman 243(2019):13-15 (1982). It has been shown that dipping freshly sawn boards in water solutions of 5% sodium sulphite or sodium thiosulfate is an effective and safe control of brown stain in eastern white pine. M. A. Hulme and J. F. Thomas, "Control of Brown Stain in Eastern White Pine with Reducing Agents," Forest Products Research Society 33(9):17-20 (1983). These processes of dipping individual pieces of sawn lumber, however, are quite time-consuming and therefore impractical for large-scale lumber manufacturing.
Other chemical treatments, such as treatment with sodium bisulfite, are under development but require two weeks of wood storage for sufficient chemical diffusion into lumber pieces. P. G. Forsyth, "Control of Nonmicrobial Sapstains in Southern Red Oak, Hackberry, and Ash Lumber During Air-Seasoning" M.S. thesis, Mississippi State University, Mississippi (1988). Further, the sodium bisulfite treatments are not effective if logs have been stored under wet conditions for more than three weeks, as is commonly done to prevent fungal attack of logs prior to processing into lumber. Iron corrosion is also a problem with this treatment. T. L. Amburgey and P. G. Forsyth, =37 Prevention and Control of Gray Stain in Red Oak Lumber," Proceedings, Hardwood Research Council pp. 92-99 (1987); P. G. Forsyth and T. L. Amburgey, "Prevention of Non-microbial Sapstains in Water-stored Oak Logs," Forest Prod. J. 42(4):59-61 (1992).